FIG. 1 illustrates drive circuits and light emitting diodes (LEDs) for prior art LED print-head (LPH) 10. FIG. 2 is a detail showing two semi-conductor chips from FIG. 1. In the example of FIG. 1, there are 40 semi-conductor chips 12 on LPH 10 and each chip includes 384 LEDs 14 forming LED array 16 of 15,360 LEDs 14. As the yield and efficiency of LED technology has improved, LPH imagers have been developed and used for xerographic printing applications, for example, in higher performance and higher quality applications. Each LED 14 is connected to a respective drive circuit 18.
FIG. 3 is a pictorial representation of portions of LPH 10 in FIG. 1. For yield reasons, optical performance and compactness, full width LPHs, i.e., LPHs spanning the entire cross process direction, are often made as multi-chip assemblies carefully assembled and focused in a housing with a SELFOC® lens, i.e., a gradient index lens or GRIN lens, as shown in FIG. 4. For clarity, the housing has been omitted in FIG. 4. SELFOC® lens 22 is arranged between multi-chip LED array assembly 16 and photoreceptor drum 24. It should be appreciated that although a photoreceptor drum is depicted in FIG. 4, other photosensitive surfaces may also be used in the foregoing arrangement, e.g., a photoreceptor belt. During xerographic printing, LED light 26 from array assembly 16 is focused on drum 24 via lens 28. The “self-focusing” property of SELFOC® lenses is well known in the art and therefore not further described herein.
FIG. 4 is a representation of clocks and a data line for LPH 10 in FIG. 1. Variation of internal performance of LEDs and LED driving circuits on each of the chips in a multi-chip LPH, which results in undesired variation of optical output for the chips, is a source of imperfect imaging for an LPH. Typically, variations of LED optical power output is corrected by per LED and/or per chip power correction in relatively small steps of optical power output, for example of 1 to 5%. Electrical power, in particular electrical current, available for energizing the LEDs is available in steps corresponding to the optical power output steps. During operation in an LPH, LEDs are energized over a range of strobe times. The strobe time is the “on” time of the LEDs during each output line time and is shown as TWSTBi in FIG. 4.
Typically, the correction, or calibration, is performed at a strobe time (calibration strobe time) that is the maximum value in the range by applying power at one of the power steps until the optical power output falls within a desired range, for example +/−2%. The electrical power, in particular, the electrical current, used for the calibration is then used for normal operation of the LEDs and LPH.
FIG. 5 is a graph depicting LED percent optical power output variation from average LPH LED optical power output for chips 12A and 12B for LPH 10 in FIG. 1 at a calibration (long) strobe time. Note that chips 12A and 12B can be located in any configuration with respect to each other in FIG. 1. FIG. 6 is a graph depicting LED percent optical power output variation from average LPH LED optical power output for the chips in FIG. 5 at a short strobe time, for example 1 microsecond (uS). Although the LPH output power can be corrected to a very good uniformity of illumination within a chip and between chips at the calibration strobe time, as shown in FIG. 5, this uniformity can degrade at different strobe times, in particular, for strobe times less than the calibration strobe time, as shown in FIG. 6. As shown in FIG. 6 there is a noticeable offset between LEDs for the first chip (left-hand side of plot) and the second chip (right-hand side of plot). This offset can result in band effects visible to the naked eye.
Returning to FIG. 4, a single master clock CLKM is used for LPH 10 and hence for all the chips on LPH 10. LED strobe CLKS is the same for every chip in LPH 10. CLKS goes high and low according to rising or falling edges of master clock CLKM, for example, edges E1 and E2, respectively. LEDs are energized and de-energized via drive circuits 18 in response to the rise and fall of CLKS, respectively. Rise time TDR on internal strobe clock CLKSI is the time span needed for CLKSI to go high (optical power output to reach a maximum) in response to CLKSI and fall time TDF is the time span needed for CLKSI to go low (zero optical power output) in response CLKSI. TDR and TDF are due to delays inherent in the circuitry of driver circuits 18 and the internal characteristics of the various LEDs 14.
If TDR and TDF of respective LED driver circuits 18 are the same, strobe time on CLKSI is equal to TWSTB on CLKS. However, if the respective TDRs and TDFs vary from chip 12 to chip 12, and do not vary an equal amount, TWSTBi strobe time can vary from chip to chip. Since the LED power is calibrated to be uniform at a given TWSTB, the calibration will not produce uniform output at all TWSTB times.
To illustrate the magnitude of uncalibrated error, take the case of maximum strobe time of 30 uS. If the TDF-TDR variation across chips in LPH 10 is +/−0.1 uS, this results in a +9.6/−9.7% chip average power variation at a TWSTB time of 1 uS since the TWSTBi time would vary by +/−0.1 uS. Even for a TDF-TDR variation of +/−0.05 uS results in a range of chip powers of +/−5%. Depending on operating exposure and xerographic transfer curve, this amount of 5% power variation may only result in less than 1-2% density variation in critical halftone densities. However, since this chip wide density band variations are very noticeable, anything greater than 0.5% or lower may not be acceptable for mid to high quality printers.
It is known to address the imaging uniformity problem describe above by specifying a minimum TWSTB time allowed during operation of the LPH, for example, one which will limit the maximum chip uniformity to some acceptable value. This solution is not ideal since 1) it still enables some level of chip wide streaks in printing even at TWSTB times at or above minimum, 2) it does not enable the very low TWSTB times needed if printing at slow speeds where lower exposure is needed for xerographic control, 3) the minimum specified time may not be sufficient for high quality printers.